This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Understanding changes in energy expenditure (EE) with aging is important in determining how to maintain a healthy weight at old age. The purpose of this proposed pilot study is to determine whether changes in EE in response to acute exercise are influenced by age in healthy, normal weight men and women. We hypothesize that a single 45-minute session of moderate-intensity exercise will result in a larger decline in spontaneous physical activity (SPA) and a smaller increase in resting metabolic rate (RMR) in older compared to middle-aged and younger individuals. We will also determine whether changes in EE in response to acute exercise are related to changes in regulatory hormones, including leptin and thyroid hormones. Five (5) men and 5 women will be recruited into one of three age groups: young, middle-aged, and old. SPA, RMR, and hormone levels will be measured before, immediately after and 24 hours after the acute exercise bout. SPA will be measured using a RT3 accelerometer and RMR will be measured using indirect calorimetry. As potential covariates, caloric intake will be analyzed using 1- and 4-day food records and body composition will be determined using DXA. Pre-exercise, post-exercise, and change values will be compared between age groups, after adjusting for confounding factors. The results from this study will begin to clarify the effects of age on changes in EE in response to exercise.